Conventional fluid valves are typically of the rotary stem or sliding stem type. A rotary valve for fluid service incorporates a rotary shaft held in a cylinder of a valve body. The shaft, upon rotation, either aligns a radial port of the shaft with a fluid port of the valve body to open a valve passage, or mis-aligns the ports to close the valve passage. In operation, the typical rotary valve stem must rotate about 90 degrees relative to the valve body between the fully open and closed positions. Other types of rotary valve designs are available that utilize alternative geometries requiring shaft rotation that is less than 90 degrees, such as three way or angled ball valves. Rotary valves typically employ seals, and often bearings, between the rotary shaft and valve body to prevent fluid from leaking from the valve body between the shaft and its cylinder. This significant movement causes substantial wear to both the seals and, if present, the bearings. Thus, the bearings and seals of such a valve must be replaced over time. The seals, in order to function properly, also add friction between the cylinder and stem. Substantial force is therefore necessary to overcome the seal friction and to rotate the stem.
A sliding stem valve operates similar to a piston and has a valve plug on a stem that slides linearly within a cylinder of a valve body. The valve plug bears against a seat or closes a passage when closed, and is spaced from the seat or clears the passage when open. Again, the valve stem and valve plug must move relative to the cylinder of the valve body a significant distance between the fully open and closed positions. Sliding stem valves typically employ seals, and often guides, between the stem and valve body to prevent fluid from leaking from the valve body between the stem and its cylinder. The significant linear movement of the valve body causes wear on the seals, and if present, the bearings, and thus the seals and bearings must be replaced over time. The seals also create friction that must be overcome in order to move the linear stem valve between the open and closed positions.